High frequency cookers mounted with a system for automatically sensing a completion of heating of a material to be heated are made to be utilized in various forms. As a sensing device used for a high frequency cooker, there are a humidity sensor for detecting a change of humidity, and a temperature sensor for detecting a change of temperature. The humidity sensor detects a change of electric resistance of an element in accordance with a water molecule adhered on the element surface, and the temperature sensor detects a temperature to be changed in response to a heated state of a material to be heated, and a heating source is controlled in accordance with the detected humidity or temperature.
However, in case of utilizing the humidity sensor, sensitivity is decreased by contamination of the element surface. Since contamination of the element surface should be periodically burnt out in order to prevent such sensitivity deterioration and in order to maintain a stable capacity, a complicated structure and operation are required. Additionally, a thermister for a temperature sensor is an expensive part. A thermister should be used in case of utilizing a temperature sensor and, hence the price of the product becomes increased. Furthermore, heating becomes uneven because a temperature error in case of actual cooking in accordance with the fixing position of thermister and distance to the heating source.